Numerous attempts have been made in the prior art to bond metal parts to glass surfaces such as rear-view mirror assemblies to the inner surface of an automobile windshield. In one known prior art method, a rear-view mirror assembly is securable to a flat metallic supporting member which is attached to the inner side of an automobile windshield by means of a thermoplastic adhesive layer using heat and pressure. After the flat metallic supporting member is attached to the windshield, the windshield is inserted into the body of the automobile and the remaining portion of the rear-view mirror assembly is fastened onto the metallic support. Examples of prior art disclosures which illustrate this method include French Pat. No. 1,226,901; U.S. Pat. No. 3,130,103; German Offenlegungsschrift No. 2,604,030; and German Offenlegungsschrift No. 2,632,164.
Because the surface of the metallic supporting member which is bonded to the glass surface is relatively small and coupled with the relatively large mechanical stresses of the rear-view mirror assembly, it has been recognized in the prior art that a firm durable bond between the metallic supporting member and the glass surface must be established.
Adhesives which have been found to perform satisfactory in the prior art include adhesive strips made of polyvinyl butyral. These adhesive strips have an excellent adhesive affinity for silicate glass, and are currently used nearly all over the world as a self-adhesive thermoplastic intermediate layer in laminated safety glass panes.
It is also known from experience that the material composition of the supporting member is of decisive importance for the quality of the bond. Moreover, it has been known that the frequently observed detachment of the metallic supporting member is due to the difference between the coefficients of thermal expansion of the metal and the silicate glass. Primarily for this reason it has been proposed that the metallic supporting member be prepared from a material whose coefficient of expansion is about the same as that of the silicate glass of the windshield. This attempted solution is essentially disclosed in French Pat. No. 1,226,901. Some improvement may be brought about also by using a relatively soft bond which is capable of equalizing the stresses produced by differential thermal expansion. However, soft bonding often proves ineffective in supporting the mirror assembly in position.
To prevent this last-mentioned defect, a method has been proposed for preparing the plate-like supporting member from silicate glass such as that disclosed also in French Pat. No. 1,226,901. While this method makes it possible to obtain a good bond, the suppporting members produced from silicate glass have many other serious drawbacks such that this proposal has not received either practical or commercial acceptance.
It has now been found that by practice of the present invention, the difficulties and disadvantages of prior art attempts to secure a rear-view mirror assembly to the surface of a glass windshield are now overcome in a simple, highly efficient manner.